Chapter 27 Human Development and Heredity

1. Chapter 27Human Development and Heredity

2. Introduction • The purpose of the reproductive system is to produce offspring whose genetic information is faithfully transmitted from generation to generation.

3. Fertilization to Birth • Fertilization • Fertilization (conception); refers to the union of an egg and a sperm. • Fertilization takes place around the time of ovulation. • Fertilization normally occurs within the fallopian tube. • The fertilized egg is called a zygote.

4. Fertilization to Birth - cont’d • Human Development • Development is a process that begins with fertilization and ends with death. • The two phases are prenatal development and postnatal development.

5. Fertilization to Birth - cont’d • Prenatal Development • Prenatal development includes four processes: cleavage, growth, morphogenesis, and differentiation. • The prenatal period consists of the early embryonic, embryonic, and fetal periods. • The early embryonic period lasts for 2 weeks after fertilization. The zygote develops and implants into the uterine endometrial lining; its trophoblastic cells secrete hCG (maintains the corpus luteum).

6. Fertilization to Birth - cont’d • Prenatal Development—cont’d • The prenatal period—cont’d • The two types of twins: monozygotic (identical) twins and dizygotic or fraternal (nonidentical) twins. • The embryonic period lasts for 6 weeks and involves the formation of the extraembryonic membranes, the placenta, and all of the organ systems. • The four extraembryonic membranes are the amnion, the chorion, the yolk sac, and the allantois.

7. Fertilization to Birth - cont’d • Prenatal Development—cont’d • The prenatal period—cont’d • The placenta develops as the chorionic villi of the embryo burrow into the endometrial lining of the uterus. The mother and embryo form two separate circulations. • The placenta has two functions: a. the site of “exchange” of nutrients and waste, and b. glandular function (estrogen and progesterone) throughout the pregnancy. • The embryo is hooked up to the placenta by the umbilical cord.

8. Fertilization to Birth - cont’d • Prenatal Development—cont’d • The prenatal period—cont’d • The embryonic period is a period of organogenesis. The organs arise from the primary germ layers: the ectoderm, the mesoderm, and the endoderm. • The fetal period extends from week 9 to birth; it is a time of growth and maturation. • Hormonal changes during pregnancy are summarized in Table 27-1.

9. Fertilization to Birth - cont’d • Birth (Parturition) • Labor consists of the forceful contractions that expel the fetus from the uterus. • Labor is caused by hormones. • The three stages of labor are the dilation stage, the expulsion stage, and the placental stage.

10. Fertilization to Birth - cont’d • Breasts and Lactation • The breasts contain mammary (milk-secreting) glands and adipose tissue. • The mammary glands and surrounding structures are affected by two hormones, prolactin and oxytocin. Prolactin stimulates the mammary glands to increase the production of milk. Oxytocin stimulates the smooth muscle in the breast to release the milk (part of the milk let-down reflex).

11. Postnatal Development • Immediate Postnatal Changes • The baby takes a first breath and the cardiovascular system makes some major adjustments. • The baby’s transition to “life on the outside” is monitored by the Apgar scale. • Developmental Periods: Neonatal Period, Infancy, Childhood, Adolescence, Adulthood, and Senescence

12. Heredity • DNA, Genes, and Chromosomes • Genetic information is stored in the DNA of genes, which are arranged into chromosomes. • Chromosomes exist in pairs. With the exception of sex cells, most human cells have 23 pairs of chromosomes, or 46 chromosomes. • Genes are dominant, recessive, or codominant. • A genetic mutation is due to a change in the genetic code.

13. Heredity - cont’d • DNA, Genes, and Chromosomes—cont’d • The sex chromosomes are designated X and Y. A female has two X chromosomes, and a male has one X chromosome and one Y chromosome. Because only the male has the Y chromosome, the father determines the sex of the child. • Sex-linked traits are carried on the sex chromosomes. Most are carried on the X chromosome and are therefore also called X-linked traits.

14. Heredity - cont’d • Congenital and Hereditary Diseases • A congenital disorder is a condition present at birth. • Congenital disorders include inherited and noninherited birth defects and diseases. A hereditary disease is transmitted genetically from parent to child.